Method and means for making a photosensitive device



S. F. ESSIG Nov. 24, 1936.

METHOD AND MEANS FOR MAKING A YHOTOSENSITIVE DEVICE Original Filed June2'7, 1934 IM LNTOB Patented Nov. 24, 1936 PATENT OFFICE METHOD AND MEANSFOR MAKING A PHOTOSENSITIVE DEVICE Sanford F. Essig, Philadelphia, Pa.,asaignor to Radio Corporation of America, a corporation of DelawareApplication June 27, 1934, Serial No. 132,570

Renewed February 6, 1936 15 Claims.

My invention relates to improvements in meth- :;is and means for makinga photosensitive de- For the purpose of developing picture signals fortelevision transmission, it has been proposed to mount a mosaic,photosensitive screen in an evacuated tube, to project onto the screenan image of the object, and to scan the screen with a cathode ray. Inthe development of these tubes, one of the problems has been in theconstruction of the screen and the step of photosensitizing the mosaicsurface thereof. The essential characteristics of such a screen are thatthe same consist of a great number of minute, conductive,photosensitized elements disposed over a surface of a supporting basemember, each element being spaced and insulated from the others. In onetype of television transmitting tube, the supporting base member for thescreen has been in the form of a thin sheet of mica provided on one sidethereof with a metallic coating to which an output connection is made,and on its other side with the mosaic of the elements referred to, eachof the photosensitive elements, with the metallic layer, constituting aminute condenser.

In operation, an image of the object is pro- ,iected onto thephotosensitive surface, causing emission of electrons therefrom anddevelopment accordingly of electrostatic charges over the surface, themagnitude of any particular charge being proportional to the lightintensity on the respective elemental area of the screen.

For most satisfactory results, it is important that the charges referredto accumulate linearly and be retained and, to this end, it is essentialthat no leakage shall occur between adjacent elements or elemental areasof the screen. In this connection, the screens referred to arephotosensitized by first oxidizing the metallic elements and thengenerating within the tube a vapor of an alkali metal, such as caesium,and in amount somewhat in excess of that required. In this way, and ifsilver is used, the silver oxide is reduced to metallic silver, theoxygen combining with the caesium to form one or a mixture of thevarious oxides of caesium so that each individual element is providedwith a coating of caesium oxide which serves to effectively bind a layerof caesium of monatomic dimensions.

The tube is then baked and pumped to remove the excess caesium, andobtain the maximum degree of sensitivity, after which the temperature isreduced to room temperature.

During the step of introducing caesium into the tube, free or looselybound caesium unavoidably condenses on the insulating mica surfacebetween the individual elements, thereby connecting them electrically.It seems that most of this caesium is removed during the baking andpumping step, but at any rate, enough remains to constitute at leastsome degree of shortcircuiting path between the adjacent elements toprevent them from storing the electrostatic charges linearly andretaining the charges. The remaining caesium could be removed bycontinued baking and pumping, but in doing this the temperature wouldhave to be relatively low to prevent removal of the desired monatomiclayer of caesium on the individual elements. Under such conditions, anundesirably long period of pumping would be necessary, during whichconditions might occur to impair the photosensitivity previouslyobtained.

With the foregoing in mind, it is one of the objects of my invention toprovide an improved method and means of making a photosensitive deviceof the character referred to whereby the insulating surface between theindividual photosensitized elements is entirely cleaned of anyshort-circuiting material which might remain after completion of thephotosensitizing step.

Other objects and advantages will hereinafter appear.

In accordance with my invention, an agent capable of absorbingphotosensitive material is placed within the tube and isolated againstabsorbing action with respect to the material. A vapor of thephotosensitive material is then generated within the tube tophotosensitize the screen, after which the tube is baked to obtain themaximum degree of photosensitivity. The agent referred to is thenreleased to absorb any photosensitive material which might have remainedon the insulating surfaces between adjacent elements whereby theresistance between the latter is made as near infinity as possible.

More specifically, in accordance with my invention, a column, to thesurface of which has been applied a coating of material capable ofabsorbing the photosensitive material, is supported within the tube. Asheet of material such as mica is wrapped about the column to cover theabsorptive material and isolate the same against absorbing action withrespect to the photosensitive material. Means including a fusibleelement in the form of a fine wire operates to hold the mica sheetwrapped about the column. After the photosensitizing step a voltage isapplied to the wire to fuse the same, whereupon the mica sheet referredto is permitted to unfold from about the column and expose the absorbingagent which is then efiective to absorb any free caesium still.remaining on the mica surface between the minute photosensitiveelements.

My invention resides in the improved method and means of the characterhereinafter described and claimed.

For the purpose of illustrating my invention, an embodiment thereof isshown in the drawing. wherein Figure 1 is an elevational view, partlybroken away, of a ath ode ray transmitting tube provided with theconstruction and means for carrying out y improved method;

Fig. 2 an enlarged, fragmentary, sectional view, the section being takenon the line 2--2 in Fig. 1;

Fig. 3 is a view similar to Fig. 2, showing the condition of-therespective parts after carrying out my improved method; and

Fig. 4 is an enlargedgfragmentary, sectional view of the mosaicphotosensitive screen, the section being taken on the line 2-2 in Fig.1.

In the drawing the reference numeral I0 designates a cathode raytransmitting tube for television, provided with a screen l2 and means I4 of conventional construction for developing a ray l6 of electrons anddirecting the ray at the screen. The screen comprisesa supporting basemember in the form of a sheet I8 of mica provided on the back surfacethereof with a metallic coating 20, and on its front surface with agreat number of minute metallic elements 22 arranged in the form of. amosaic and each spaced from the others. The screen is supported at itsfour corners by wires 24 extending outwardly from "a column 26 supportedwithin the tube from a reentrant portion 28 of the wall structure. 4

Each of the metallic elements 22 is oxidized to provide the same with acoating 30 of the oxide of the metal. In carrying out thephotosensitizing step. a capsule 60 previously placed within the tube iscarried by the supporting wire ill attached to the wire 52, andcontaining an alkali metal such as caesium, is exploded to generate avapor of the alkali metal. A coating 32 of caesium oxide is thus causedto form on each of the elements 22 and a photosensitive layer of caesiumis bound to this coating. During this step free or loosely boundcaesium, indicated at 34 in Fig. 4, condenses on the mica surfacebetween the individual elements 22 to connect them electrically, therebymaking it impossible for these elements to retain an electrostaticcharge.

In the next step of photosensitization, the tube is baked andsimultaneously pumped to diminish the photosensitive caesium layer tothe point of maximum photosensitivity. During the baking and pumpingstep most of the free caesium previously condensed on the mica surfacebetween the elements 22 is evaporated off, but enough of this freecaesium remains to provide at least some degree of leakage paths betweenthe individual photosensitized elements to prevent them from retaining agiven charge for a fraction of a second. Continued pumping has not beeneffective to remove all of this small amount of free caesium.

For the purpose of removing or cleaning up any remaining free caesium onthe mica surface between the elements 22, I propose to apply to thesurface of the column 26 a coating 36 of an agent such as lead oxide orstannic oxide capable of abto wires 60 sorbing the caesium. A sheet 88of mica is wrapped about the column 26 to cover the absorbing agent andisolate the same against absorbing action with respect to the caesium.This sheet is held about the column 26 by complementary metal parts 40and 4| bent and embracing the column 26 as shown in Fig. 2. These partsare held together about the column 26 by a sheet 42 of mica, for whichpurpose they are threaded as shown through slits 44 in the sheet 42. andtheir respective ends are bent around and secured nd 62, respectively,to which these ends are welzed. The wire 60 is bent as shown and isconnected at 64 to the metallic coating 20 of the screen l2, from whichthis wire extends outwardly through the wall of the tube III to providean output connection from the tube. The wire 52 is bent as shown andextends through the reentrant portion 28- of the tube for the purposehereinafter explained.

The complementary parts 40 and 4| are also held in position about thecolumn 26 by a fine fusible wire 56 connected to the other ends of theseparts. One end of the wire 68 is welded directly to the end of the part4|, while the other end of this wire is wrapped about and welded to anL-shape metal piece 58 welded to the end of the part 46.

The mica sheet 38, due to its resiliency and tendency accordingly tounfold from about the column 26, might split at its free edge 60 wherethe part 40 presses against the sheet. To prevent this, it is proposedto weld a short piece 62 of wire to the end of the part 40, this pieceof wire extending axially of the column 26 and along the free edge 60 ofthe mica sheet.

During the photosensitizing step explained above, the mica sheet 36remains wrapped about the coating 36 of lead oxide or stannic oxide sothat this agent is not effective to absorb any of the caesium vapor.After this step, a voltage is applied to the connections 50 and 52 tofuse the wire 56 and permit the parts 40 and 4| to spring outwardly andthe mica sheet 38 to unfold from about the column 26, as shown in Fig.3, whereupon the coating 36 is exposed to the atmosphere within the tubeIll and is effective to absorb all of the free caesium which might haveremained on the mica surface between the elements 22 or on othersurfaces within the tube l0. At this time the tube is at room tempera.ure or somewhat higher, but not sufliciently gh to cause evaporation ofthe photosensitive caesium layers on the elements 22.

From the foregoing it will be seen that by my improved method and meansit is possible to employ a clean-up agent after completion of thephotosensitizing step, the agent being isolated and ineffective duringthis step. An important feature of my improved means resides in the factthat the absorbing or cleaning-up action of the agent for this purposeis controlled externally of the tube I 0, by making connections to thewires 50 and 52.

While my improved method and means has been described more in connectionwith the construction of a cathode ray tube for television transmission,it has a much broader adaptation, and can be applied in the constructionof different types of photosensitive devices where it is desired toremove a certain amount of photosensitive material after thephotosensitizing step has been completed.

While but one embodiment of my invention has been disclosed, it will beunderstood that various modifications, within the conception of thoseskilled in the art, are possible without departing from the spirit of myinvention or the scope of the claims.

I claim as my invention:

1. Aphotosensitive device comprising a tube provided with electrodes oneof which is photosensitive, an agent disposed on an interior surface ofsaid tube and capable of absorbing photosensiing said agent t isolatethe same against absorbing action th respect to said material, and meansfor removing said covering means to expose said agent to the atmospherewithin said tube.

2. A photosensitive device comprising a tube provided with electrodesone of which is photosensitive, an agent disposed on an interior surfaceof said tube and capable of absorbing photosensitive material, meanscovering the surface containing said agent to isolate the same againstabsorbing action with respect to said material, and means holding saidcovering means against said surface.

3. A photosensitive device comprising a tube provided with electrodesone of which is photosensitive, a member supported within said tube andprovided on part of its surface with a coating of material capable ofabsorbing photosensitive material, a sheet of material wrapped aboutsaid member and covering said surface to isolate said agent againstabsorbing ,action with respect to the photosensitive material, and meansholding said sheet wrapped about said member.

4. A photosensitive device comprising a tube provided with electrodesone of which is photosensitive, a column supported within said tube andprovided on part of its surface with a coating of material capable ofabsorbing photosensitive material, a. sheet of material wrapped aboutsaid column and covering said surface to isolate the absorptive agentagainst absorbing action with respect to the photosensitive material,and fusible means holding said sheet wrapped about said column.

5. A photosensitive device comprising a tube provided with electrodesone of which is photosensitive, a column supported within said tube andprovided on part of its surface with a coating of material capable ofabsorbing photosensitive material, a sheet of material wrapped aboutsaid column and covering said surface to isolate the absorptive agentagainst absorbing action with respect to the photosensitive material,complementary parts embracing said column and holding said sheet wrappedabout the same, and fusible means connected to adjacent ends of saidparts and holding the latter in position about said column.

6. A photosensitive device comprising a tube provided with electrodesone of which is in the form of a screen comprising a supporting base andminute photosensitive elements carried thereby and disposed in mosaicfashion over said base and spaced from each other, a member supportedwithin said tube and provided on part of its surface with a coating ofmaterial capable of absorbing photosensitive material from the surfacesof said base between the individual photosensitive elements, a sheet ofmaterial wrapped about said member and covering said surface to isolatesaid agent against absorbing action with respect to the photosensitivematerial, and means holding said sheet wrapped about said member.

7. A photosensitive device comprising a tube tive material, miafnscovering the surface containprovided with electrodes one of which is inthe form of a screen comprising a supporting base and minutephotosensitive elements carried thereby and disposed in mosaic fashionover said base and spaced from each other, a member supported withinsaid tube and provided on part of its surface with a coating of materialcapable of absorbing photosensitive material from the surfaces of saidbase between the individual photosensitive elements, a sheet of materialwrapped about said member and covering said surface to isolate saidagent against absorbing action with respect to the photosensitivematerial, complementary parts insulated from each other and embracingsaid member and holding said sheet wrapped about the latter, fusiblemeans connected to adjacent ends of said elements and holding the latterin positionrabout saidmember, and electrical connections from therespective ends of said fusible means and extending through the wallstructure of said tube to the exterior thereof for applying a voltagesufflcient to cause fusing of said fusible means.

8. A photosensitive device comprising a tube provided with electrodesone of which is photosensitive, an agent disposed within the interior ofsaid tube and capable of absorbing photosensitive material, meanscovering the said agent to isolate the same against absorbing actionwith respect to said material, and means for removing said coveringmeans to expose said agent to the atmosphere within said tube.

9. A photosensitive device comprising a tube provided with electrodesone of which is photosensitive, an agent disposed within the said tubeand capable of absorbing photosensitive material, means covering thesaid agent to isolate the same against absorbing action with respect tosaid material, and means holding said covering means against said agent.

10. A photosensitive device comprising a tube provided with electrodesone of which is photosensitive, a member supported within said tube andprovided on part of its surface with a coating of material capable ofabsorbing photosensitive material, a sheet of material wrapped aboutsaid member and covering said surface to isolate said agent againstabsorbing action with respect to the photosensitive material, andreleasable means holding said sheet wrapped about said member.

11. A photosensitive device comprising a tube provided with electrodesone of which is in the form of a screen comprising a supporting base andminute photosensitive elements carried thereby and disposed in mosaicfashion over said base and spaced from each other, a member supportedwithin said tube and provided on part of its surface with a coating ofmaterial capable of absorbing photosensitive material from the surfacesof said base between the individual photosensitive elements, a sheet ofmaterial wrapped about said member and covering said surface to isolatesaid agent against absorbing action with respect to the photosensitivematerial, and releasable means holding said sheet wrapped about saidmember.

12. A photosensitive device comprising a tube provided with electrodesone of which is. photosensitive, a source of supply of photosensitivematerial, an agent disposed within the interior of said tube and capableof absorbing photosensitive material, means covering the said agent toisolate the same against absorbing action with respect to said material,and means for removing said covering means to expose said agent to theatmosphere within said tube.

13. A photosensitive device comprising a tube provided with electrodesone of which is photosensitive, a source of supply of photosensitivematerial, an agent disposed within the said tube and capable ofabsorbing photosensitive material, means covering the said agent toisolate the same against absorbing action with respect to said material,and means holding said covering means against said agent.

-14. A photosensitive device comprising a tube provided with electrodesone of which is photosensitive, a source of supply of photosensitivematerial, a member supported within said tube and provided on part ofits surface with a coating of material capable of absorbingphotosensitive material, a sheet of material wrapped about said memberand covering 'said surface to isolate said agent against absorbingaction with respect to the photosensitive material, and releasable meansholding said sheet wrapped about said member.

15. A photosensitive device comprising a tube Provided with electrodesone of which is in the form of a screen comprising a supporting base andminute photosensitive elements carried thereby and disposed in mosaicfashion over said base and spaced from each other, a source 0! supplyoi' photosensitive material, a member supported within said tube andprovided on part of. its surface with a coating of material capable ofabsorbing photosensitive material from the surfaces of said base betweenthe individual photosensitive elements, a sheet of material wrappedabout said member and covering said surface to isolate said agentagainst absorbing action with respect to the photosensitive material,and releasable means holding said sheet wrapped about said member.

SANFORD F. ESSIG.

